The present invention relates to an assembly of a semiconductor device such as IC (integrated circuit) chips or, more particularly, to an assembly of an IC chip and a circuit board electrically connected at the terminals of the IC chip to the electrodes of the circuit board with high reliability despite the simple and inexpensive structure for their connection which enables reduction of the thickness of the assembly.
IC chips are usually used as being electrically connected at their terminals or so-called bonding pads with electrodes of a circuit board which is prepared by patterning the copper foil on a copper-foiled laminated plate such as glass cloth-reinforced epoxy resin laminates, glass cloth-reinforced polyimide resin laminates, polyimide resin films and the like. The most conventional method for electrically connecting the electrodes on the circuit board and the terminals of the IC chip is the so-called wire-bonding method in which a bonding pad of the IC chip and an elecvtrode on the circuit pattern are electrically connected with a very fine wire of gold, aluminum and the like. This method is, however, disadvantageous in respect of the incompatibility with the requirements in the modern semiconductor technology. For example, the increase in the density of integration in the IC chip necessarily results in the number of the bonding pads so that the utilizability of the IC chip can be ensured only by decreasing the size of the individual bonding pads and the pitch of arrangement of the bonding pads. Consequently, an extremely high precision is required in the positioning of the bonding wires and the working time is increased for bonding of an increased number of the bonding wires one by one necessarily leading to an increase in the production costs.
An alternative method for electrically connecting an IC chip and a circuit board, called the flip-chip method, has been developed to solve the above mentioned problems caused by the increase in the density of integration. In this method, each bonding pad on the IC chip is provided with a protrusion or bump formed of a solder alloy and the electrods on the printed circuit board are contacted with these solder bumps to establish electric connection between the respective bonding pads and electrodes. This method is somewhat advantageous over the wire-bonding method in the simplified working process without the individual bonding of the bonding wires but involves several problems. For example, the electric connection cannot be uniform among the sets of the bonding pads and electrodes when the number of the bonding pads is increased depending on the forms of the solder bumps and, in particular, on the planarity of the circuit board. Decrease in the width of the space between adjacent solder bumps requires certain measures to prevent current leakage therebetween due to adsorption of moisture. Moreover, the difference in the thermal expansion is not negligible between a large IC chip and the printed circuit board to cause stress at the contact points of the solder bumps and the electrodes on the circuit board resulting in eventual destruction of the electric connection. Therefore, this method of flip-chip bonding is not always quite satisfactory in respect of the poor reliability of the electric connection.
Further, a method called the TAB method is known and practiced, in which the electric connection between bonding pads of an IC chip and electrodes of a circuit board is obtained by means of a leading wire formed by etching a copper foil adhesively bonded on a continuous-length polyimide film. In this method, a number of leading wires can be exactly positioned together with the IC chip and circuit board and the work time for the electric connection is not unduly long in addition to the advantage that the above mentioned stress caused by the difference in the thermal expansion can be absorbed by the lead wire. This method is, however, disadvantageous because each bonding pad on the IC chip must be provided with a bumped electrode by plating or like method or alternatively the contacting point of the lead wire with the bonding pad must be shaped in the form of a protrusion by etching or like method so that the yield of acceptable products necessarily decreases with consequently increased costs.